It has long been known that Sagnac effect may be utilized to detect rotation rate of various obJects. See references: e.g., Post review of Modern Physics, Vol. 39 p. 475 1967; and V. Vali, R. W. Shorthill and M. F. Berg, Applied Optics Vol. 15, p. 1099, 1977. Also, refer to U.S. Pat. Nos. 3,102,953, and 4,013,365 for disclosures relating to apparatus for measuring angular rotation. The Sagnac gyroscope has been developed vigorously, especially since low loss long length single mode optical fibers became available.
One of the technical difficulties in optical gyroscope technology stems from the fact that unless .pi./2 non-reciprocal phase bias scheme is utilized, the gyro sensitivity is at its minimum near zero rotation rate. And most of the gyro applications deal with measurement of near zero rotation rate. There are various ways of implementing the .pi./2 nonreciprocal phase bias in the optical fiber loop in order to overcome the problem. However, they add to the gyro system additional optical noise system complication, and device size and cost. The present invention provides an optical fiber gyroscope with a three fiber directional coupler for coupling light into opposite ends of a fiber loop to traverse the loop in mutually opposite directions and for receiving the light from opposite respective fiber ends and combining it in a manner whereby optical output is at quadrature or most sensitive to change at near zero rotation of the loop.